Industrial waste liquid may contain organic matter, as a byproduct, such as formaldehyde or methanol. Such waste liquid is discharged after it is treated to make the concentration of formaldehyde fit a waste liquid standard. As a method of treating formaldehyde or methanol in the waste liquid, the activated sludge method is used, in many cases.
In general, the preferable water temperature of the activated sludge method is considered to range from 20° C. to 30° C., as described, for example, in “Haisuishori no Seibutsugaku (Biology of Waste Liquid Treatment)”, written by Sudo, published by Sangyo Yosui Chosakai (Jun. 12, 1977). When the temperature exceeds 35° C., the activity of microorganisms in activated sludge deteriorates to some extent.
Specifically, JP-A-11-19685 (“JP-A” means unexamined published Japanese patent application), JP-A-11-19686, JP-A-9-253696, and JP-A-7-232178 disclose methods of treating formaldehyde-containing waste liquid, in which the period of acclimatization can be made short or is not required. Among those, JP-A-11-19685 and JP-A-11-19686 describe methods of using a specific microorganism capable of decomposing high-concentration formaldehyde. Further, JP-A-9-253696 and JP-A-7-232178 describe methods of decomposing formaldehyde without using any activated sludge. The methods described in JP-A-9-253696 and JP-A-7-232178, which are different from any activated sludge method, require the setting up of facilities and catalysts, to raise the temperature and pressure of the waste liquid to be treated.
On the other hand, as to facilities for waste liquid treatment in the activated sludge method, JP-A-8-24885 discloses a method of using a microfiltration membrane filter, thereby omitting a sedimentation tank for sludge, which is conventionally necessary in the activated sludge method, and saving on facility-installation space.
Further, in European Patent No. 0 699 172, a membrane bioreactor (MBR) for treating waste liquid which contains a hydrocarbon is disclosed.
Incidentally, in recent years, the demand for water has been increasing, based on a rising population, and the amount of water that can be used per person has been increasing, based on an improvement in the level of living, while new development of water (fresh water) resources has not been pursued. Further, the demand for water that can be used to produce food has also been increasing, from the rise in population. As a result, insufficiency of water resources has been coming to the fore as a big problem.
In particular, water reuse has been actively practiced in regions where new water resources are hard to obtain.
Under such circumstances, stringent countermeasures against water shortage problems have been demanded also in industrial fields where relatively large amounts of water are consumed. In plants, water is used in various systems, as necessary utilities for the process of producing products, such as boiler feed water, cooling water, or general service water. Since the reduction of the consumption of such valuable water is directly related with the reduction of production costs, active reuse of waste liquid has been practiced or studied in each plant.
More specifically, waste water or sullage discharged (formed) following reactions in chemical plants or the like, is treated for reuse as boiler feed water or general service water in plants, industrial complexes, or the like. However, waste water formed in chemical plants and water generated by chemical reactions are often discharged at a temperature of 100° C. or higher without any appropriate treatment. In such cases, the waste water cannot be biologically treated without appropriate treatment, because the temperature suitable for the activity of aerobes (bacteria) in activated sludge used for biological treatments (hereinafter simply referred to as “active temperature”) is generally in the range of from 10° C. to 35° C. Therefore, under the status quo, such waste water must be cooled to a temperature of at least about 30° C.
However, when biologically treated waste liquid is reused as boiler feed water in a plant, the water must be reheated to a temperature of 100° C. or higher. More specifically, there is a disadvantage of poor energy efficiency for the whole process of cooling waste liquid at a temperature of 100° C. or higher down to about 30° C. for biological treatment and reheating the resultant water liquid to 100° C. or higher.
In areas where the ambient temperature is high, such as the Middle East, the lowest temperature which can be reached by cooling with seawater, which is a low-cost cooling medium, is 40° C., and further cooling requires a particular cooling medium and installation of a freezer.